When parking a vehicle in a parking lot, it is difficult to recognize the locations of obstacles and other parked vehicles, and high driving skills are thus necessary to park a vehicle. Hence, various types of parking assistance devices have been proposed to assist parking. Patent document 1 describes an example of a parking assistance device that uses an ultrasonic sensor and a vehicle speed sensor to detect the distance from the vehicle to a parked vehicle located near the vehicle before the vehicle starts to move in reverse. The parking assistance device stores detected distance data and travel distance data of the vehicle. The parking assistance device calculates the length of a parked vehicle in the travel direction of the vehicle from the travel distance data. When doing so, the parking assistance determines the size of the parked vehicle to be shorter than the parked vehicle length that is obtained by integrating the detected distance data and sets a parking target region (parking space). The parking assistance device obtains the positional relationship with the parking target region to perform driving assistance for parking.
In the parking assistance device of patent document 1, the width of the parked vehicle is calculated to be shorter than the detected parked vehicle length for reasons that will now be discussed. The above-described ultrasonic sensor has a detection range having the shape of a fan or a long oval. Thus, an obstacle such as a parked vehicle is detected as long as it is located in the detection range even when the obstacle is not located in front of the ultrasonic sensor.
In this manner, the shortening of the calculated length of the detected obstacle (parking vehicle), that is, the correction made so that the calculated length becomes close to the actual length of the obstacle, allows for the parking space to be accurately calculated to a certain degree. However, the actual size of the obstacle is not accurately detected. Thus, even when corrections are made, the parking space may not be accurately calculated. Accordingly, there is a demand for a parking space monitoring device that calculates the parking space with higher accuracy.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-7875